As is known, epicyclic gearings are widely used in the field of aeronautic engines for transmitting drive and converting power between a turbine engine (having high speed and low torque) and at least one propulsive element (having high torque and low speed), as they are very efficient for implementing this function whilst keeping weight and bulk down.
Similar design solutions exist outside of the aeronautics industry, especially in the aerogenerator construction field, where the gearing instead performs the function of a speed multiplier and not a reducer.
In addition to the natural saving in weight, the need to reduce the bulk of the gearing as much as possible is particularly felt in the new aeronautic engine architectures being studied for reducing consumption and pollution (architectures such as non-direct-driven types of turbo fan and open rotor). In these architectures, epicyclic gearing is actually integrated with the turbine engine, where the diametral envelope of the gearing tends to condition the geometries of the passageways for the flow of air or combustion gases, and therefore decisively affects the efficiency of the turbine engine.
One advantageous solution for implementing these gearings contemplates using a planet-carrier having an annular plate that supports two arrays of planet gears, arranged on opposite sides of the plate. In particular, the planet gears are mounted with bearings on respective pins, which protrude from the plate in opposite directions parallel to the axis of the gearing. Solutions of this type are known, for example, from EP2339208 and WO2013065024, in the name of the applicant.
Within the field of this type of solution, a strong need is felt to seek a perfectly equal distribution of the loads along the various torque transfer paths in the gearing. This equal distribution is a necessary condition for achieving maximum lightness and minimum overall bulk for the gearing. In fact, potential load maldistribution and the uncertainty in estimating this maldistribution imposes using overload factors in the design and sizing of the components of the gearing (cogwheels, bearings, etc.), independently of what their effective stress state is, with consequent oversizing of all the parts, even those that are less stressed in practice.
Load maldistribution is essentially due to the following factors:                an epicyclic gearing, also depending on the particular solution adopted, may become hyperstatic, and therefore the torque transfer paths are inevitably subjected to greater stress where greater rigidity is provided;        with respect to the nominal assembly and geometric conditions, the gearing inevitably has constructional and assembly errors and tolerances that cause relative displacements between the various components with respect to that planned by design and therefore generate overloads;        asymmetries in the loads/displacements can possibly be imposed from the outside, at the connection interfaces of the gearing.        
In general, the main solutions capable of minimizing the first two of the above-indicated factors can be classified as follows:                solutions that aim at minimizing the geometrical differences (asymmetries) along the various torque transfer paths and at introducing orientable elements or flexible elements to support the planet gears, for example of the so-called flexpin type (where the pins that support the planet gears have the ability to bend in a localized manner); and        solutions that aim at compensating the asymmetries by an opportune balancing of the rigidity on the various torque transfer paths.        
The solution described in WO2013065024 effectively reduces load maldistribution by introducing radial joints to support the plate. However, this solution needs a relatively large number of components and the joints are subject to wear on the friction-coupled parts, and therefore have low reliability.
It is therefore preferable to direct design towards solutions where the planet-carrier has no joints.
In this regard, in the solutions described in patents EP2072863 and EP2072858, the planet-carrier is advantageously made in a single piece.
However, these last two solutions have a considerable asymmetry of the planet-carrier with respect to the plane of symmetry of two arrays of planet gears and do not contemplate the introduction of flexible elements of the flexpin type to support these planet gears.
In particular, the single-piece planet-carrier is constituted by a plate that supports the two arrays of planet gears, via an attachment portion connected to a rotating shaft or to a static structure, and by arms or beams that connect the plate to the attachment portion. These arms are substantially parallel to the axis of the gearing and are positioned in the spaces between the planet gears, in a circumferential direction. Torque is thus transferred, from the plate to the above-mentioned arms and from the latter to the attachment portion.
This architecture is unsatisfactory, as the arms, as well as deforming under load, tend to transfer the deformations that are naturally present in the attachment portion during use to the plate. As a result, the plate is subjected to a bending moment imparted by the arms at the respective connection points. The bending of the plate, resulting from these localized bending moments, causes the axes of the pins supporting the planet gears to tilt, thus generating an undesired unbalancing, i.e. maldistribution, of the loads on the planet gears between one and the other of the two arrays and undesired reaction stress arises in the connection zone between the pins and the plate.
To limit this unbalancing, caused by the deformation of the attachment portion, and hence contain the stress generated by the bending moments in the plate, the latter must be made with a substantial axial width. However, this sizing makes the planet-carrier particularly sensitive to gearing construction and assembly errors and tolerances (for example, displacements of the position of the planet gears with respect to that planned by design). This sensitivity to errors and tolerances results in further load maldistribution and significant overloads on the planet gears.
Therefore, the solutions described in EP2072863 and EP2072858 are not able to effectively compensate for the asymmetry of the planet-carrier in order to limit load maldistribution.
Moreover, the particular solutions proposed in EP2072863 and EP2072858 do not allow making the support pins of the planet gears in one piece with the plate, as, by doing so, it would not be possible to mount the planet gears and associated bearings. Therefore, these solutions require couplings with high interference between the carried pins and the plate in order to prevent the significant risk of wear on the parts in contact.